Generally, the hydrophobicity of a porous carbon substrate may be obtained by impregnation of a fluorinated polymer of polytetrafluoroethylene type (PTFE).
This type of hydrophobic treatment may be particularly adapted for certain elements of a fuel cell, particularly a gas diffusion layer, a gas diffusion electrode, a microporous layer, an active layer, or the catalyst support.
It may be carried out by treating the substrate with an emulsion of fluorinated polymer such as PTFE, by:
a/ immersing the substrate into a solution based on PTFE;
b/ drying the impregnated substrate in air;
c/ repeating steps a/ and b/ to reach the desired PTFE concentration;
d/ sintering the dried substrate at a temperature higher than the glass transition temperature of PTFE, generally at approximately 350° C.
Other variations have been envisaged, for example, the use of polyethersulfone or of fluorinated polymers other than PTFE.
Anyhow, this type of treatment generally requires sintering the substrate, thus increasing the general energy cost.
On the other hand, the hydrophobic polymer may be distributed in uncontrolled fashion, particularly when the substrate is made of carbon fibers. Indeed, in the case of the impregnation of a solution based on PTFE, the latter concentrates on the external surfaces of the substrate with a deficiency at the heart thereof.